Recently, a broadband internet connection environment progresses rapidly. Along with this progress, it is necessary to further increase transmission capacity of a network in the whole area from a core system to an access system or metropolitan area system. Therefore, a large-capacity network in which optical communication technology is utilized is actively developed.
In the optical communication light source unit, the light emitting device such as a laser diode (LD) is used to convert an electric signal into an optical signal. At this point, in order to secure the normal optical communication, it is necessary that the optical output wavelength and optical output power of each light emitting device constituting a light source unit should be set at values determined according to determined optical wavelength arrangement and optical transmission path loss while controlled so as to be maintained at the values. The optical output wavelength and optical output power of each light emitting device depend on the drive current and device temperature, and are uniquely determined in a normal operating range.
In the conventional technique, the setting and control of the optical output wavelength and optical output power at the determined values are realized by the use of a combination of means for automatically controlling the device temperature, means for automatically controlling the optical output power, and means (wavelength locker) for automatically controlling the optical output wavelength.
However, the wavelength locker is extremely expensive, and the extremely complicated setting and control are required, which becomes large barriers to application to the access system or metropolitan area system in which low cost and simplification are required.
Non-Patent Document 1 describes an example of the conventional method of setting the optical output wavelength and optical output power of the light emitting device such as LD. The outline of Non-Patent Document 1 will be described below.
FIG. 1 shows a configuration example of the conventional optical communication light source unit.
The optical communication light source unit includes a light emitting device 11, means 12 for automatically controlling the optical output power of light g from light emitting device 11 such that the optical output power is maintained at a given target value, means 13 for automatically controlling the device temperature of the light emitting device 11 such that the device temperature is maintained at a given target value, means 14 for branching the light from the light emitting device 11, and means (wavelength locker) 15 for automatically controlling the optical output wavelength of the branched light h such that the optical output wavelength is maintained at a given target value. In the light source unit, the setting and control of the optical output wavelength and optical output power of the light emitting device 11 are performed as follows.
In the wavelength locker 15, as shown in FIG. 2(b), photocurrent Im generated by detecting the light is periodically changed for the optical output wavelength λ. In the conventional technique, the setting and control of the optical output wavelength are performed by utilizing the characteristics of the photocurrent Im.
Specifically, the device temperature is coarsely adjusted using the means 13 for automatically controlling the device temperature such that the device temperature is maintained at a given target value as shown in FIG. 2(a), and thereby the optical output wavelength λ is put in a pull-in range 25 shown in FIG. 2(b) corresponding to optical output separately specified wavelength λs of the wavelength locker 15. Then, the means 12 for automatically controlling the optical output power such that the optical output power is maintained at a given target value is simultaneously operated to set and control the optical output power at the separately specified value while the optical output wavelength is maintained in the pull-in range 25. Finally, the wavelength locker 15 and the means 13 for automatically controlling the device temperature such that the device temperature is maintained at a given target value are simultaneously operated while the control target value of the means 12 for automatically controlling the optical output power such that the optical output power is maintained at a given target value is fixed to the specified value.
At this point, the optical output wavelength λ is precisely adjusted to the specified value by finely adjusting the photocurrent generated in the wavelength locker 15 to the value corresponding to the separately specified optical output wavelength λs.
Thus, in the conventional technique, because the setting and control of the optical output wavelength and optical output power of the light emitting device 11 are performed while divided in plural steps, the setting and control become extremely complicated, and the extremely expensive wavelength locker 15 is required.
Even if the optical output wavelength is precisely adjusted to the specified value, the optical output power is fluctuated due to aged deterioration and the like when the light emitting device is continuously used for a long time. Means for automatically controlling the optical output power (Automatic Power Control Circuit: APC) is activated in association with the fluctuation in optical output power. As a result, the drive current of each light emitting device is also fluctuated such that the optical output power is maintained at the determined value. This causes the optical output wavelength of each light emitting device to be fluctuated outside an allowable range of the determined optical wavelength arrangement.
In the conventional optical communication light source unit shown in FIG. 1, the following control is performed against the aged deterioration of the light emitting device. That is, in the wavelength locker 15, the photocurrent Im generated by the detection of the light is periodically changed for the optical output wavelength λ as shown in FIG. 2(b), so that the optical output wavelength is controlled by utilizing the characteristics of the photocurrent Im. Non-Patent Document 2 describes an example of the conventional method of controlling the optical output wavelength and optical output power of the light emitting device such as LD.
Specifically, the wavelength locker 15 and the means 13 for automatically controlling the device temperature such that the device temperature is maintained at a given target value are simultaneously operated while the control target value of the means 12 for automatically controlling the optical output power such that the optical output power is maintained at a given target value is fixed to the specified value. Therefore, when the optical output power of the light emitting device 11 is fluctuated, the means 12 for automatically controlling the optical output power such that the optical output power is maintained at a given target value is operated to change the drive current of the light emitting device 11 such that the optical output power becomes the control target value. The change in drive current changes the optical output wavelength and device temperature of the light emitting device 11. As a result, the wavelength locker 15 and the means 13 for automatically controlling the device temperature such that the device temperature is maintained at a given target value are operated to return the optical output wavelength to the specified value, and control the device temperature such that the device temperature is maintained according to the drive current in the optical output power fluctuation.    Non-Patent Document 1: “Power Source™ Tunable High Power CW Laser Module with Integrated Wavelength Monitoring”, [online], Avanex, Inc., [search on Jul. 23, 2004], the Internet TLI.APP.pdf>    Non-Patent Document 2: Proceedings of the 2002 Institute of Electronics, Information, and Communication Engineers General Conference No. C-4-44, Takagi, et al. “25 GHz-spacing interval wavelength monitor integrated DFB laser module” P. 349 (2002)    Non-Patent Document 3: A. Zadok, et al. “Spectral shift and broadening of DFB lasers under direct modulation”, IEEE Photon. Technol. Lett., Vol. 10, No. 12, pp. 1709-1711, 1998    Non-Patent Document 4: ITU-T recommendation, G959. 1, 2001